ActionScript for Flash MX: The Definitive Guide, Second Edition

This is the Title of the Book, eMatter Edition

Copyright © 2007 O’Reilly & Associates, Inc. All rights reserved.

The Almighty Prototype Chain

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311

A custom setSuperClass( ) function

If you decide to use __proto__ to establish inheritance (despite Macromedia’s recom-

mendations against it), consider creating a custom function that hides the slightly

obtuse

__proto__ syntax. Example 12-6 shows a custom setSuperClass( ) function

that assigns a superclass to a subclass (i.e., establishes inheritance) using

__proto__.

Note that we might have naturally used the name “extends” instead of “setSuper-

Class” (following Java terminology), but “extends” is reserved as a potential keyword.

A word of warning to

__proto__ users: when inheritance is established via

prototype.__proto__, Flash Player 6’s super operator cannot be used to invoke a

superclass constructor. The superclass constructor must be invoked with the follow-

ing older style syntax if inheritance was established using

__proto__:

function SubClass (someArg) {

this.base = SuperClass;

this.base(someArg);

delete this.base;

}

Example 12-6. A setSuperClass( ) function

// Add setSuperClass() to Function.prototype so it's

// available to all function objects

Function.prototype.setSuperClass = function (theSuperClass) {

// Establish the super/sub class relationship.

this.prototype.__proto__ = theSuperClass.prototype;

}

// Create a superclass

function Circle (radius) ...